CONDITION,
.......Some customers may comment on a harsh 1-2 up-shift and the Check Engine Light or Service Engine Soon indicator is illuminated.
Diagnosis,
.......Typically, these vehicles will have been driven more than 32,000 km (20,000 mi) before this condition occurs.
.......The scan tool may show a DTC P1870 set as a history code.
........A Harsh 1-2 Shift or DTC P1870, caused by wear in the..."Control Valve Body", may be difficult to duplicate when the transmission temperature is below 93°C (200°F).CAUSE,
......The condition may be due to wear in the control valve body. This wear occurs in the bore that contains the TCC isolator and regulator valves, and results in poor, or no, TCC apply

IMPORTANT,
......DTC P1870 is a type B code. The conditions for setting the P1870 DTC must occur on TWO CONSECUTIVE TRIPS (ignition cycles, with a drive cycle) before setting a P1870 history code.
......When the conditions for setting DTC P1870 are met (first trip), the PCM commands maximum line pressure and harsh 1-2 shifts are the result.
......This may result in a harsh 1-2 shift with no history code if the conditions for setting the DTC required for the second trip are not met, on two consecutive trips (Ignition cycles, with a drive cycle).
......When the conditions for setting the DTC are met, on the second consecutive trip, a DTC P1870 is stored as a history code.
......When the P1870 code is stored, the PCM will turn on the Service Engine Soon (SES), ......Check Engine Light (CEL), or Malfunction Indicator Lamp (MIL).

CORRECTION,
Install a control valve body with the revised TCC regulator and isolator valves. These valves are used in all transmissions produced after January 15, 1999 (Julian Date 9015), and all of the service parts currently available through GMSPO contain revised TCC regulator and isolator valves.

Also,
......If all of the following conditions are true, it is not necessary to rebuild the transmission or to replace additional transmission components beyond the "Control Valve Body".
.......Transmission operation is normal before the transmission reaches operating temperature, or before DTC P1870 is set (no slips, flares, or missing gears).
......The torque converter is not blue or overheated.
......The transmission fluid is not burned or has no burned odor.
......The transmission fluid pan contains no abnormal debris (clutch material, bronze, brass, or metal fragments).

Had my 2011 Reg Cab 4x4 4.8 4l60E harsh 1-2 shift fixed today. Although my check engine light did not come on, I did retrieve a P0961 code, that is after I went out and bought a OBD2 reader that was CAN enabled. Anyway it turned out to be connector X175 not seated properly. Likely any other trucks that went by the same assembler may have had the connector not properly seated.

i have the same issue with hard 1-2 shift and i have the new control valve body to replace it with. My 2000 chevy 1500 C/K 4X4 has an issue i have noticed where at around 50 mph with accelerator depressed just enough to maintain constant speed my engine surges a little, maybe 100-500 rpm. it will surge and surge every few seconds untill i either speed up or let off the accelerator. will replacing the control valve body fix this problem too?

Mine is a 93 k1500 with the 4L60E and it shifts hard between 1st and 2nd could it be the valve body in mine
it is consistant whether hot or cold other then that shifts smoothly other gears
if so where can i get the updated valve body
thanks Frank